In last batch of meatballs that I made, I followed a tip that I'd found in some other recipe and used grated summer squash as a filler instead of the bread crumbs my recipe called for. It actually worked pretty well. My family did ask what the yellow flakes were in the meatballs (it did look a little strange), but they also said that they liked them very much. They were my best meatballs yet. I'd tried almond meal and flax as a filler -- but got a thumbs down from my family on those meatballs.